Reduction of NOx is of increasing concern as emission guidelines become more stringent. Types of emission treatments may include nitrogen oxide storage catalysts, also known as NSR catalysts, (e.g., NOx storage and reduction catalyst) or a lean NOx trap (LNT) may represent two exemplary aftertreatment devices for the catalytic conversion of nitrogen oxides. An alternative technology may include a selective catalytic reduction (SCR) device, which may utilize a reductant solution applied thereon to reduce nitrogen oxides.
LNT catalysts may store nitrogen oxides at temperatures below a threshold, wherein the nitrogen oxides stored thereon may be reduced in the presence of a rich air/fuel mixture. The hydrocarbons and carbon monoxide may function as reducing agents. Contrastingly, the SCR device may increase reduction efficiency at temperatures above the threshold, resulting in the SCR and LNT working in tandem to treat nitrogen oxide emission at a greater range of exhaust gas temperatures.
Thus, to meet the more stringent emission guidelines, it may be desired to pair the LNT catalyst and the SCR device in an exhaust gas treatment arrangement to increase a temperature range in which the nitrogen oxides may be reduced. However, this may lead to problems. In one example, the rich mixture used to reduce nitrogen oxides captured by the LNT catalyst may leak through the LNT, wherein the rich exhaust gas mixture may reach the SCR device, which may decrease the longevity of the SCR device.
One example approach to address rich exhaust gas flow to the SCR device is shown by Choung in U.S. 2017/0167337. Therein, an air supply device is arranged upstream of the SCR catalyst. If a combustion mixture is rich (e.g., lambda less than 1), air is supplied to the exhaust gas flow via the air supply device to increase the air/fuel ratio to greater than 1.
However, the inventors herein have recognized potential issues with such systems. As one example, the supply of air may decrease exhaust gas temperatures, which may decrease an efficiency of the SCR device and decrease nitrogen oxides reduced. Additionally, the air may oxidize a reductant agent arranged on the SCR device or being injected thereto, thereby increasing reductant agent consumption.
In one example, the issues described above may be addressed by a system for a lean-NOx-trap arranged upstream of a selective-catalytic-reduction catalyst in an exhaust passage. An air supply device is arranged between the lean-NOx-trap and the selective-catalytic-reduction catalyst. The system further comprises a controller with computer-readable instructions stored thereon that when executed enables the controller to adjust an air flow from the air supply device when an exhaust gas is rich and an exhaust gas temperature is greater than a threshold. In this way, air from the air supply device only flows when the exhaust gas temperature is greater than the threshold.
As one example, by allowing rich exhaust gas to flow to the SCR when its temperature is less than the threshold, the SCR may continue to treat nitrogen oxides during the current engine operating parameter or a later engine operating parameter. Furthermore, reductant stored on the SCR or being delivered to the SCR may be preserved, decreasing reductant consumption.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.